The present invention relates to leakage test equipment which measures leakage of gas or liquid from a variety of vessels and decides whether the measured leakage is within a given limit.
In a conventional leakage test in a production line of vessels which are demanded to meet the requirement that the leakage of gas or liquid therefrom be within a fixed value, a vessel under test (hereinafter referred to as a work) and a leak-free reference tank (hereinafter referred to as a master tank) are coupled together, for instance, via a diaphragm type differential pressure sensor and charged with gas under the same test pressure, and then the pneumatic circuit is closed. After a certain elapsed time, the differential pressure between the work and the master tank is read and converted to the leakage for the work. It is decided whether the work is good or unacceptable, depending upon whether the converted value is within a given limit or not.
FIG. 1 illustrates a typical arrangement of such conventional leakage test equipment and one complete cycle of the leak test for each vessel by the equipment is carried out through a sequence of steps shown in FIG. 2, under control of a microcomputer 25 in an operation controller 20. After setting a pneumatic pressure from a compressed air source 11 to a test pressure by a pressure reducing valve 12 while reading pressure indications on a pressure gauge 44, a work to be tested (i.e. a vessel) 16 is connected to a measuring-side pipe 42 in a rest period of step S.sub.1. In a charging step S.sub.2 an instruction is issued from the CPU 25A via an output port 25E to a drive power supply 39 to activate it, by which a three way electromagnetic valve 13 is communicated with a measuring pipe circuit 23 to supply air via normally open two way electromagnetic valves 14, 15 to the work 16 and a master tank 17 and, at the same time, a first timer (not shown) set in a RAM 25C is started. After a certain elapsed time T.sub.1, a balancing step S.sub.3 is initiated, in which pneumatic systems on the work side and the master side now put under the same test pressure are closed by the two way electromagnetic valves 14 and 15, respectively, and at the same time, a second timer (not shown) is started. Accordingly, if air leaks out of the work 16, the differential pressure across a diaphragm type differential pressure sensor 18 will increase with the lapse of time. In the event that a relatively large leakage is detected in this time period, it is decided that the work 16 is defective, and the process immediately returns to step S.sub.1. After the lapse of predetermined time T.sub.2 during which such a large leakage has not been detected, the process proceeds to a detection step S.sub.4, in which a third timer (not shown) is set in the RAM 25C and the differential pressure between the work 16 and the master tank 17 is detected as an electric signal by the differential pressure sensor 18 and amplified by an amplifier 19 for an indication by a pointer A.sub.1 of a meter relay type indicator 21. Accordingly, the differential pressure thus indicated corresponds to the leakage from the work 16 after closure of the two way electromagnetic valves 14 and 15 in step S.sub.3. Where this differential pressure exceeds an allowed value set by a setting needle A.sub.2 of the meter relay type indicator 21, it is decided that the work 16 is unacceptable, and an alarm 45 is actuated by the output of the meter relay 21. The process immediately returns to step S.sub.1, in which the work 16 is replaced by the next work, for which the same test cycle is repeated. The test sequence is executed under control of the CPU 25A in accordance with a program stored in a ROM 25B.
Incidentally, the relationship between the leakage .DELTA.V.sub.L from the work 16 and the differential pressure .DELTA.P after closure of the two way electromagnetic valves 14 and 15 in FIG. 1 can be obtained in such a manner as described below. Symbols for use in equations are defined as follows:
P: Pressure in the work and master tank at the start of detection, that is, a test pressure (Kg/cm.sup.2 G) PA1 P.sub.W : Pressure (Kg/cm.sup.2 G) in the work at the end of detection, i.e. after the lapse of time T.sub.3 PA1 P.sub.M : Pressure (Kg/cm.sup.2 G) in the master tank at the end of detection PA1 V.sub.W : Inner volume (cc) of the pneumatic system on the work side at the start of detection PA1 V.sub.M : Inner volume (cc) of the pneumatic system on the master tank side at the start of detection PA1 K.sub.S : Volume change ratio of the diaphragm type differential sensor to pressure variations, i.e. the sensitivity of the diaphragm to the differential pressure (cc/Kg/cm.sup.2) PA1 K.sub.W : Volume change ratio of the work to pressure variations (cc.Kg/cm.sup.2) PA1 K.sub.M : Volume change ratio of the master tank to pressure variations (cc/Kg/cm.sup.2) PA1 .DELTA.V.sub.L : Leakage from the work (atm cc) PA1 T.sub.3 : Detection time (sec) PA1 G: 1 atm=1.03 Kg/cm.sup.2
According to Boyle's law, the following equations hold: EQU (1.03+P)V.sub.W =(1.03+P.sub.W)[V.sub.W -K.sub.S .multidot..DELTA.P-K.sub.W (P-P.sub.W)]+1.03.multidot..DELTA.V.sub.L ( 1) EQU (1.03+P)V.sub.M =(1.03+P.sub.M)[V.sub.M +K.sub.S .multidot..DELTA.P-K.sub.M (P-P.sub.M)] (2)
Letting P.sub.M .apprxeq.P in the differential pressure .DELTA.P=P.sub.M -P.sub.W at the end of detection, .DELTA.P.apprxeq.P-P.sub.W. Using them, Eqs. (1) and (2) are changed as follows: EQU P.multidot.V.sub.W .apprxeq.P.sub.W .multidot.V.sub.W -(K.sub.S +K.sub.M)(1.03+P.sub.W).multidot..DELTA.P+1.03.multidot..DELTA.V.sub.L ( 3) EQU P.multidot.V.sub.M .apprxeq.P.sub.M .multidot.V.sub.M +(1.03+P.sub.M).multidot.K.sub.S .multidot..DELTA.P (4)
From Eqs. (3) and (4) the leakage .DELTA.V.sub.L is expressed as follows: ##EQU1## In Eq. (5) V.sub.E and K are defined as follows: ##EQU2## Accordingly, the leakage .DELTA.V.sub.L is given as follows: EQU .DELTA.V.sub.L =(V.sub.E /1.03).multidot..DELTA.P (8)
The term V.sub.E is called an equivalent inner volume of the pneumatic system on the work side, and the term K is called the volume change ratio of the work side pneumatic system (cc/Kg/cm.sup.2). The differential pressure per unit leakage (atm cc), that is, .DELTA.P/.DELTA.V.sub.L, is the leakage detection sensitivity of this leakage test equipment, and this is given in the form of 1.03/V.sub.E (Kg/cm.sup.2 /atm cc).
Since K.sub.S, K.sub.W and V.sub.M can be regarded as constant in Eq. (6), the equivalent inner volume V.sub.E can be regarded as a function of the test pressure P and the inner volume V.sub.W of the work. Since the presure P and the inner volume V.sub.M differ with the types of works, the equivalent inner volume V.sub.E differs accordingly. The inner volume V.sub.E can be obtained in the following manner: In FIG. 1, a leak-free work is connected as the work 16, and a volume changer 24 is connected to the work side pneumatic system 42. The volume changer 24 is a syringe type device with a micrometer in which its inner volume 27 can be changed by turning a knob 33 to displace a piston in a cylinder. The amount of volume changed corresponding to the number of turns of the knob 33 is preknown. The leakage test sequence in FIG. 2 is executed and when the sequence reaches step S.sub.4, the knob 33 is turned a predetermined number of times to move back the piston, causing a volume change .DELTA.V (an increase). Since this increased volume is equivalent to leakage, as viewed from the work side pneumatic system, the volume change .DELTA.V can be converted by the following equation, according to Boyle's law, to the volume under the atmospheric pressure, that is, the equivalent leakage .DELTA.V.sub.L. EQU .DELTA.V(1.03+P)=.DELTA.V.sub.L .times.1.03 (9)
Subsitution of Eq. (9) into Eq. (8) gives EQU V.sub.E =(.DELTA.V/.DELTA.P).times.(1.03+P) (10)
Accordingly, by causing the volume change .DELTA.V, and measuring the differential pressure .DELTA.P, the equivalent inner volume V.sub.E corresponding to the work being connected can be calculated from Eq. (10) provided that the test pressure P is given. It is therefore necessary, in the prior art, to precalculate the equivalent inner volume V.sub.E for each work through use of Eq. (10) and to calculate an allowed differential pressure .DELTA.P' corresponding to an allowable leakage .DELTA.V.sub.L ' for each type of work, through use of Eq. (8). In the leak test, it is required that the above-mentioned allowed differential pressure .DELTA.P' be re-set on the meter relay type indicator 21 each time the type of work is altered.
For checking the leak detection sensitivity (1.03/V.sub.E) of the leakage test equipment, a predetermined volume change is provided by the volume changer 24 as in the case of measuring the afore-mentioned equivalent inner volume, and it is checked whether the differential pressure indicated at that time stays within a predetermined range. Where the differential pressure is out of the predetermined range, it is judged that the sensitivity error is large, the equivalent inner volume V.sub.E for the work used is calculated from Eq. (10), and the value thus obtained is substituted into Eq. (8) to calculate the allowed differential pressure .DELTA.P' corresponding to the allowable leakage .DELTA.V.sub.L '. The leakage detection sensitivity is similarly checked for each type of work, and if necessary, the allowed differential pressure .DELTA.P'. For the type of a work formed to have a large error as a result of the leak test, a newly calculated allowable differential pressure is set on the meter relay 21 in the subsequent test for each work of the same type; and for the type of a work decided not to have a large error, the allowed differential pressure calculated in the preceding test for a work of the same type is set on the meter relay 21 in the subsequent test for the same type, thereby calibrating the leak detection sensitivity. Such checking and calibration of the leak detection sensitivity calls for a very cumbersome operation of manually turning the micrometer of the volume changer 24 a predetermined number of times for each type of work in the detection step S.sub.4, as described above.
As described previously, the leak test employs the allowable leakage as the criterion for deciding whether the work is good or bad. Since the leakage is measured by detecting the differential pressure, the allowable leakage is converted by Eq. (8) into the corresponding allowable differential pressure, which is set by the setting needle A.sub.2 of the meter relay 21. As will be seen from Eq. (8), however, since the equivalent inner volume V.sub.E differs with the type of work, it is necessary to set the allowable differential pressure on the meter relay 21 for each type of work. Accordingly, in the case where different types of works are supplied to the leak test equipment through different channels, the allowed differential pressure must be set manually for each channel selection; this is very troublesome.
It is therefore an object of the present invention to provide leakage test equipment which permits direct reading of the measured leakage of a work and directly compares the leakage with the allowable leakage for deciding whether the work is good or unacceptable, and hence is free from the necessity of converting the leakage into the corresponding allowable differential pressure.
Another object of the present invention is to provide leakage test equipment which facilitates the checking and calibration of the leak detection sensitivity for each type of work.